Tensile-stress fracturing

ABSTRACT

In fracturing a subsurface earth formation, an oriented tensile stress that is focused in a plane that is perpendicular to the axis of a borehole is utilized to initiate a horizontal fracture. The stress is produced by notching the walls of the borehole to form diverging conical surfaces and detonating an explosive substantially midway within the confined section between the notches. The explosion propagates a semicylindrical compressive wave and is reflected from the conical surfaces as a pair of tensile waves. The waves then intersect on a horizontal plane and generate a tensile force which is focused in the vertical direction. The net result is a horizontal fracture along the plane of intersection of the two reflected tensile waves.

United States Patent McLamore [4 1 Jan. 25, 1972 [54] TENSILE-STRESS F RACTURING [21] Appl.No.: 842,852

3,464,490 9/1969 Silverman ..166/247 X 3,050,122 8/1962 Huitt et al ..166/558 3,533,471 10/1970 Robinson ..l66/299 3,456,589 7/1969 Thomison et al 166/299 UX Primary Examinerlan A. Calvert Attorney-Thomas R. Lampe and .l. H. McCarthy 5 7] ABSTRACT In fracturing a subsurface earth formation, an oriented tensile stress that is focused in a plane that is perpendicular to the axis of a borehole is utilized to initiate a horizontal fracture. The stress is produced by notching the walls of the borehole to form diverging conical surfaces and detonating an explosive substantially midway within the confined section between the notches. The explosion propagates a semicylindrical compressive wave and is reflected from the conical surfaces as a pair of tensile waves. The waves then intersect on a horizontal plane and generate a tensile force which is focused in the vertical direction. The net result is a horizontal fracture along the plane ofintersection of the two reflected tensile waves.

PATENTED M25 I972 FIG. 3

lNVENTOR:

ROY T. Mc LAMORE HIS ATTORNEY TENSILE-STRFSS FRACTURING The present invention relates to the fracturing of a subsurface earth formation to facilitate the production of liquids therefrom. More particularly, the invention concerns itself with an approach whereby the explosive effect of the charge utilized in carrying out the fracturing operation is maximized to create a horizontal fracture in the subsurface earth formation.

It is a common practice to increase the productivity of oil bearing or similar earth formations by creating artificial fractures therein so as to provide passages for facilitating flow from the formations into wellbores from which the fluids can be recovered. In accordance with one approach, a hydraulic fracturing liquid is forced down the well and into the formation under sufficient hydraulic pressure to overcome the tensile strength of the formation. The fracturing fluid induces a crack into the formation and such crack is extended substantial distances into the formation by the continued application of pressure through the fluid. A problem with hydraulic fracturing treatments of this sort is that there is little control of the direction and orientation of the fracture. Under the pressure of the hydraulic fluid, the fracture is ordinarily initiated and extended along a vertical plane in the formation. Horizontal hydraulic fracturing can occasionally be induced at shallow depths less than 2,000 feet.

A second commonly used approach has been to lower an explosive device down into the wellbore with the subsequent detonation of the device forming the formation fractures. Once again, however, it is difficult to control the direction of the fractures and the force of the explosion itself is not'effectively utilized since it extends in all directions from the point of explosion. Unless extremely high-explosive forces are used, such fracturing, even if it does occur in the desired direction, may not penetrate sufficiently deep enough into the formation to enhance its productivity. In the event even higher explosive forces are used to overcome this deficiency, casing or other equipment associated with the well may be damaged during the explosion.

SUMMARY OF THE INVENTION It is therefore a primary object of the present invention to provide an improved and economical approach whereby the forces resulting from the detonation of an explosive charge in a well borehole may be more effectively utilized to fracture a subsurface formation.

It is another object of the present invention to provide an improved technique whereby a highly oriented tensile stress field perpendicular to the axis of the borehole is created, so that a horizontal fracture is formed in the surrounding earth formation in an efficient manner. v

The above and other objects have been attained in the present invention by providing an approach for fracturing a subsurface earth formation wherein an oriented tensile stress that is focused perpendicular to the axis of a borehole is used to initiate a horizontal fracture. The stress is produced by notching the walls of a borehole as by means of an expansible under-reaming cutter to form diverging conical surfaces and detonating an explosive device within a confined section substantially midway between the notches. The explosion creates a semicylindrical spherical compressive wave that is reflected from the conical surfaces as a pair of tensile waves. The waves then intersect on a horizontal plane and generate a tensile force which is focused in the vertical direction. The net result is a horizontal fracture along the plane of intersection of the two reflected tensile waves. The horizontal fracture may be extended dynamically by hydraulic means along the horizontal plane.

DESCRIPTION OF THE DBAWING The above noted and other objects of the present invention will be understood from the following description taken with reference to the accompanying drawing. In describing the invention in detail, reference will be made to the drawing in which like reference numerals designate like parts throughout corresponding views in which:

FIG. 1 is a diagrammatic view of apparatus for fracturing a subsurface fonnation in accordance with the teachings of the present invention prior to the detonation of the explosive charge;

FIGS. 2 and 3 are similar to FIG. 1 but illustrate the wave and fracture activity in the subsurface formation after detonation of the subsurface device;

FIG. 4 is a schematic illustration of the stress field vectors induced upon detonation of the explosive device; and

FIG. 5 is a diagrammatic view in longitudinal projection illustrating the operation of hydraulic means used to extend the horizontal fracture initiated by means of the detonation of the explosive device.

Referring now to FIG. 1, a well borehole 11 is illustrated as extending downwardly into a producing formation 12. A conventional well casing 13 is positioned within borehole 11 and is cemented within the well by conventional procedures whereby the well casing is surrounded by a sheath of cement 14 extending upwardly within the annulus created between the borehole and the casing outer surface.

After casing 13 has been cemented into position, a plug 15, which may be of any suitable type, is fixedly positioned within the casing 13 a suitable distance below the point where formation fracturing is desired. After plug 15 has been installed, a conventional perforator mechanism (not shown) is lowered into the casing to perforate the casing and surrounding cement, as at 16 and 17, at a location or depth where fracturing of the subsurface formation is desired. The perforations provide communication between the interior of the casing and the subsurface formation in the usual manner. After the desired perforations are formed an expansible under-reaming cutter or other suitable tool for example, a formation notching apparatus of the type described in US. Pat. No. 3,2l l,22l to I-Iuitt, (not shown) is lowered within the casing to cut upper notch 21 and lower notch 22 in the casing, the surrounding cement and subsurface earth formation. Lower notch 22 is positioned above plug 15 and below perforations 16 and 17. Notch 21, on the other hand, is positioned above perforations 16 and 17. After notches 21 and 22 have been formed, an explosive charge 25 is lowered within the casing and positioned therein in line with perforations 16 and 17, as well as any other perforations (not shown) which have been formed in the casing and the surrounding cement. A second plug 26 is then positioned within casing 13 at a point lying above upper notch 21.

For reasons which will be brought out more fully below, the shape of notches 21 and 22 is quite important, insofar as carrying out the teachings of the present invention is concerned. As may readily be seen, notches 21 and 22 are in the form of truncated cones with the base of the imaginary cone defined by notch 21 extending upwardly and the base of the imaginary cone defined by notch 22 extending in a downward direction. The conical surfaces defined by notches 21 and 22 are disposed at an angle a with respect to the horizontal, which, in order to carry out efficiently the teachings of the present invention should be 45 or less.

The operation of the above-disclosed arrangement is as follows. When fracturing is desired, explosive device 25 is detonated. Device 25 is preferably a shaped charge in accordance with well-known practices whereby substantially the full force of the explosion is at right angles to the casing throughbore axis. In other words, most of the explosive force will proceed outwardly from device 25 and through the perforations in the surrounding casing and cement, e.g., perforations 16 and 17. The size of the charge needed to create the horizontal fracture is a function of the type of explosive used, the coupling efficiency between the explosion and the borehole, the in-situ compressive vertical stress that must be negated (i.e., the depth), the tensile strength of the formation and the stress wave attentuation characteristics of the rock.

The initial result of such explosion is to create pockets of nibble in the areas of the subsurface formation surrounding the perforations in the casing and cement. In FIG. 2, for example, the rubble pockets associated with perforations 16 and 17 are respectively designated by means of reference numerals 31 and 32. These pockets of rubble are created by semicylindrical-spherical compressive shock waves which are initiated in the subsurface formation surrounding the point of detonation. Although it is to be understood that such shock waves are initiated through each of the casing perforations, in the interest of clarity the stress wave action immediately surrounding only one perforation, i.e., perforation 17, will be described in detail. The stress wave actions initiated through all the perforations extending through casing 13 will, of course, have an overlapping and cumulative effect to extend the still-to-bedescribed fracturing activity about the desired extent of the casing periphery.

The representative compressive stress or shock wave created in subsurface formation 12 due to the explosive force proceeding outwardly through perforation 17 is illustrated by means of reference numeral 36 with the direction of the shock wave being indicated by means of the arrows associated therewith. The wave force extends outwardly from perforation 17. As the compressive stress wave propagates outwardly as shown, portions of the wave 36 strike notches 21 and 22. Since the surfaces 37 and 38 of formation 12 defining the inner limits of notches 21 and 22, respectively, are free" surfaces, the portions of the compressive shock wave striking these surfaces are reflected as tensile waves 41 and 42 with the direction of the tensile forces created by such waves being indicated by means of arrows 37A and 38A. These tensile waves are focused back toward the radial plane in which the explosive charge 25 was originally detonated. Tensile waves 41 and 42 intersect along the plane of detonation and reinforce one another in such a manner that a horizontal fracture is created along said plane. Dynamically induced horizontal fractures 45 and 46, which resulted from explosive waves emanating from perforations l6 and 17, respectively, are illustrated in FIG. 3. Such fractures lie along the plane of detonation and extend further into formation 12 than would be the case where only compressive waves are utilized in such fracturing operation. To assist in the further analysis of the wave action resulting from an explosion with the instant arrangement, reference may be had to FIG. 4 which illustrates the stress field resulting in the subsurface formation at the instant of intersection of the tensile waves and the compression wave. It may be seen that the force vectors P, which result from the tensile waves are disposed in opposition to one another and at right angles to the resultant compression wave force vector F which lies along the plane of detonation. This stress field configuration serves to substantially expand and lengthen the fracture created by detonation of the explosive device 25. The fracture created by detonation of the explosive device may, if desired, be further dynamically extended and in FIG. one suitable system for this purpose is illustrated. In the disclosed system three pressure cells are established within casing 13 prior to detonation of explosive device 26. To accomplish this, a pressure rupture disk 51 is fixedly positioned within casing 13 so that a pressure P is established in the zone lying between the rupture disc 51 and the explosive device 26. Pressure P is set below the fracture extension pressure P which is established from field tests. A second pressure cell is established within casing 13 between rupture disc 51 and a housing element 52 positioned thereabove within the casing in a fluidtight manner. Housing element 52 has a fluid flow passageway 53 therein and further accommodates a ball check valve 54 which is adapted to selectively terminate fluid flow within passageway 53 in an obvious manner. A pressure, P is maintained within the above-described second pressure cell prior to detonation of explosive device 26. Pressure P is set considerably above fracture extension pressure P Finally, a pressure cell is established in the portion of the casing lying above housing element 52. The pressures P and P are maintained within their respective portions of the easing by means of suitable surface pressure pumps (not shown),

which are operatively associated with conduits extending into each cell zone. To simplify matters, only one such conduit i.e., conduit 50, has been shown. P is generally set at the normal value of the hydrostatic pressure at the particular depth of the subsurface formation being fractured. Pump pressure P is set above Pp but below P such that,

When explosive device 25 is detonated, the generated gases will raise P until the pressure rupture disc 51 fails. Thus,

When this situation occurs, the fracture initiated by the above-described dynamic stress waves will begin to extend due to hydraulic loading. As the fracture extends and the fluid penetrates the fracture, the pressure P;, will eventually drop below pressure P,, and the ball valve 54 will reverse. In such a manner surface pumps (not shown) can be brought into play and the hydraulic pressure at the face of the fracture may be maintained well in excess of the fraction extension pressure. Once the surface pumps are brought into use, the fracture can be extended and propped by the usual methods.

The system illustrated in FIG. 5 can, if desired, be used to create multiple horizontal fractures. If adequate pumping power is available, a series of horizontal fractures could be initiated and extended simultaneously. If such pumping power is not available, multiple fractures could be created singularly by starting at the lowest desirable production level and working up the hole.

I claim as my invention:

1. A method of forming a substantially horizontal fracture in a subsurface earth formation penetrated by a well to increase the rate of flow of formation fluids into the well, said method comprising:

mechanically cutting upper and lower vertically spaced notches into the subsurface earth formation defining said well, said upper and lower notches each having an upper and a lower surface,

the lower surface of the upper notch defining truncated cone coaxial with the well having a substantially vertical longitudinal axis and being oriented with its base extending upwardly,

the upper surface of the lower notch defining a truncated cone coaxial with the well having a substantially vertical longitudinal axis and being oriented with its base extending downwardly,

the lower surface of the upper notch and the upper surface of the lower notch each defining an angle of 45 or less with respect to a substantially horizontal plane; thereafter detonating an explosive device in said well at a point between said upper and lower spaced notches whereby a compressive shock wave is initiated in the subsurface formation surrounding said point of detonation;

and whereby tensile waves are created in said subsurface formation by the interaction of said lower surface of said upper notch and upper surface of said lower notch with the compressive forces of said wave portions;

said tensile waves being reflected from said surfaces back toward a substantially horizontal radial plane which intersects said point in the well at which the explosive device was detonated whereby said reflected waves intersect along said radial plane to produce a substantially horizontal fracture extending into said subsurface formation along said radial plane a substantially greater distance than said upper and lower notches extend into said subsurface earth formation.

2. The method of claim 1 wherein said fracture in said subsurface formation is hydraulically extended after being formed by the interaction of said waves.

3. The method of claim 1 wherein plug means are fixedly positioned within said well above said upper notch and below said lower notch prior to detonation of said explosive device.

4. The method of claim 1 wherein said well includes a casing extending into said subsurface formation and wherein said casing is perforated along said substantially horizontal radial plane prior to detonation of said explosive device, said perforations being effective to direct said compressive wave along said substantially horizontal radial plane and toward said notches.

5. The method of claim 1 including the steps of u establishing first and second pressure cells within the well, the first pressure cell including a portion of the well which extends from below the lower notch to above the upper notch and the second pressure cell including the portion of the well immediately adjacent the top of the first pressure cell, the first and second pressure cells being separated by a pressure rupture disc; adjusting the pressure in the first pressure cell to a pressure less than the fracture extension pressure of the formation; supplying a fluid to the second pressure cell at a pressure greater than the fracture extension pressure of the formation; detonating the explosive device, thereby generating gases which raise the pressure in the first pressure cell and cause the rupture disc to fail; thereby opening the first pressure cell into fluid communication with the second pressure cell; and

continuing to supply fluid to the second pressure chamber at a pressure greater than the fracture extention pressure of the fonnation to hydraulically extend the substantially horizontal fracture into said subsurface formation.

6. The method of claim 5 wherein the step of establishing first and second pressure cells comprises 

1. A method of forming a substantially horizontal fracture in a subsurface earth formation penetrated by a well to increase the rate of flow of formation fluids into the well, said method comprising: mechanically cutting upper and lower vertically spaced notches into the subsurface earth formation defining said well, said upper and lower notches each having an upper and a lower surface, the lower surface of the upper notch defining a truncated cone coaxial with the well having a substantially vertical longitudinal axis and being oriented with its base extending upwardly, the upper surface of the lower notch defining a truncated cone coaxial with the well having a substantially vertical longitudinal axis and being oriented with its base extending downwardly, the lower surface of the upper notch and the upper surface of the lower notch each defining an angle of 45* or less with respect to a substantially horizontal plane; thereafter detonating an explosive device in said well at a point between said upper and lower spaced notches whereby a compressive shock wave is initiated in the subsurface formation surrounding said point of detonation; and whereby tensile waves are created in said subsurface formation by the interaction of said lower surface of said upper notch and upper surface of said lower notch with the compressive forces of said wave portions; said tensile waves being reflected from said surfaces back toward a substantially horizontal radial plane which intersects said point in the well at which the explosive device was detonated whereby said reflected waves intersect along said radial plane to produce a substantially horizontal fracture extending into said subsurface formation along said radial plane a substantially greater distance than said upper and lower notches extend into said subsurface earth formation.
 2. The method of claim 1 wherein said fracture in said subsurface formation is hydraulically extended after being formed by the interaction of said waves.
 3. The method of claim 1 wherein plug means are fixedly positioned within said well above said upper notch and below said lower notch prior to detonation of said explosive device.
 4. The method of claim 1 wherein said well includes a casing extending into said subsurface formation and wherein said casing is perforated along said substantially horizontal radial plane prior to detonation of said explosive device, said perforations being effective to direct said compressive wave along said substantially horizontal radial plane and toward said notches.
 5. The method of claim 1 including the steps of establishing first and second pressure cells within the well, the first pressure cell including a portion of the well which extends from below the lower notch to above the upper notch and the second pressure cell including the portion of the well immediately adjacent the top of the first pressure cell, the first and second pressure cells being separated by a pressure rupture disc; adjusting the pressure in the first pressure cell to a pressure less than the fracture extension pressure of the formation; supplying a fluid to the second pressure cell at a pressure greater than the fracture extension pressure of the formation; detonating the explosive device, thereby generating gases which raise the pressure in the first pressure cell and cause the rupture disc to fail; thereby opening the first pressure cell into fluid communication with the second pressure cell; and continuing to supply fluid to the second pressure chamber at a pressure greater than the fracture extention pressure of the formation to hydraulically extend the substantially horizontal fracture into said subsurface formation.
 6. The method of claim 5 wherein the step of establishing first and second pressure cells comprises disposing lower plug means in said well below said lower notch; disposing intermediate plug means compriSing a pressure rupture disc in said well above said upper notch; and disposing upper plug means in said well above said intermediate plug means, said upper plug means comprising check valve means positioned to shutoff flow upwardly through the upper plug means when the pressure in the well immediately below the upper plug means is greater than the pressure in the well immediately above the plug means. 